Current progress in automotive technology is leading to the development of anti-freeze compositions which provide extended life protection against corrosion or pitting of metal surfaces. In the drive to manufacture automobiles which are more energy efficient, an increased number of metal surfaces in automobile engines are made of light weight aluminum. Examples of engine parts made of aluminum surfaces include cylinder heads, radiator cores and water pump housings and fittings. In the cooling system, aluminum corrosion products, particularly aluminum oxide, which circulate to and deposit on internal radiator surfaces, interfere with the heat transfer necessary to keep the engine from overheating. Aluminum surfaces are also particularly susceptible to localized pitting corrosion, which can perforate tubing. This corrosion problem is aggravated since engines have been designed to operate at higher temperatures. Overall, these and other factors have recently placed more stringent demands upon the performance of the aqueous glycol-based "antifreeze" compositions employed as engine coolants.
The automotive industry is also headed towards filling factory automobiles with extended life coolants so that the coolants need only be changed every 5 years and 100,000 to 150,000 miles. As a result, the demands placed upon coolants today require that the coolant have an extended life without depleting the corrosion inhibitor packages during service. While some conventional anti-freeze compositions adequately inhibit corrosion of metal surfaces, their corrosion inhibitor packages are consumed during service, requiring a change of the coolant at shorter intervals.
The corrosion inhibitor packages in conventional antifreeze coolants typically comprise borates which function to protect solder and brass surfaces, and also function as act buffers; silicates for protecting cast iron and aluminum surfaces, and in particular cavitation of aluminum water pumps; primary and secondary amines, which in combination with phosphoric acids protect aluminum and cast iron surfaces; and phosphates to buffer the anti-freeze concentrate, preserve the pH of the concentrate, and protect steel, cast iron, and aluminum surfaces.
While each of these compounds provide a measure of protection and utility, each suffers drawbacks. For example, silicate compounds, known as an ingredient in antifreeze formulations which protect aluminum surfaces, are often thermally unstable and sensitive to pH, thereby reducing their shelf life. During service, silicate compounds also tend to gel in the presence of other salts in the formulation. Each of these drawbacks decrease the coolant's effectiveness against aluminum corrosion and deplete the content of active corrosion inhibiting silicates throughout the life of the coolant. Not only is the corrosion inhibiting life of silicates decreased, but the precipitates which form during service can also abrade metal surfaces.
Phosphates suffer the drawback in that they tend to combine with silicates to cause gelling problems. Further, phosphates increase the BOD or COD levels in rivers, lakes, and other bodies of water, tending to increase the levels of algae growth. Primary and secondary amines, while know for their ability to protect aluminum and cast iron when combined with phosphoric acids, tend to also combine with nitrites to form the toxic nitrosamines. In conventional fully formulated coolants, borates tend to corrode aluminum surfaces over time.
U.S. Pat. No. 4,851,145 describes the use of an alkylbenzoic acid such as the highly preferred p-tertbutyl benzoic acid in combination with C.sub.8 -C.sub.12 aliphatic monobasic acids and hydrocarbyl triazoles as agents to protect against corrosion of metal surfaces as measured by a Rapid Cyclic Potentiokinetic Polarization Scanning technique. This and other publications, such as JP 59208082 and U.S. Pat. No. 5,489,391 and U.S. Pat. No. 2,832,742, recommend the use of p-tertbutylbenzoic acid. However, p-tertbutylbenzoic acid is toxic and is a suspected carcinogen. Its use in consumer anti-freeze applications is not favored. Further, we found that a p-tertbutylbenzoic acid containing formulation did not provide adequate resistance to corrosion of an aluminum coupon as measured in a potentiodynamic polarization test, while benzoic acid formulations exhibited a clear and sharp resistance to current flow as the potential was increased. These results and the benefits provided by benzoic acid over p-tertbutyl benzoic acid are described more fully below.
In U.S. Pat. No. 3,573,225, combinations of amine salts of fatty acids, alkali metal salts of benzoic acid, and alkanolamides have been proposed as corrosion inhibitors. Also, in DE 3,223,940, combinations of the salts of C.sub.6 -C.sub.10 aliphatic carboxylic acids, salts of C.sub.6 -C.sub.10 polyhydroxycarboxylic acids, and salts of aromatic monocarboxylic acids were proposed as corrosion inhibitors. While such salts of acids may provide suitable resistance against corrosion of many metal surfaces in fully formulated anti-freeze concentrates, we have found that many formulations containing salts of an unsubstituted aromatic carboxylic acids, such as sodium benzoates, were not effective to provide adequate protection against corrosion, particularly of iron and aluminum surfaces, in formulations free of borates, primary and secondary amines, phosphates, and silicates (BAPS free). It would be desirable to formulate an anti-freeze composition which provides adequate corrosion resistance without the need to add other corrosion inhibitors to the composition, such as the borates and silicates.
In coating applications unrelated to anti-freeze coolants for internal combustion engines, U.S. Pat. No. 4,342,596 discloses compositions containing C.sub.8 -C.sub.20 aliphatic monocarboxylic acids, aminoalkylalkanolamines, aromatic carboxylic acids, and lubricant oils which are rolled or sprayed onto metal surfaces to protect them against oxidation during storage and provide a measure of lubrication. For purposes of the present invention, formulations containing lubricating oils or secondary amines are unsuitable for the desired anti-freeze application. Likewise, EP-20,042 discloses a coating composition applied onto metal surfaces to protect the surfaces against oxidation, provide lubrication, and which can be washed off prior to painting, which composition comprises 5-20 pbw of an aliphatic monobasic acid, optionally a lubricant, 10-35 pbw of an aromatic carboxylic acid, and an amine forming water soluble salt with the acid components. Similar compositions which are rolled, dipped, or sprayed onto metal surfaces and again washed off when the metal surface is ready to be painted or coated can be found in U.S. Pat. No. 3,573,225. Each of these compositions, however, are aqueous based compositions or alcohol based compositions, and each contain water soluble amine salts of the acids or primary or secondary amines.
Although stable and otherwise compatible inhibitor combinations have been developed, they are, in general, either unduly expensive, objectionable from the standpoint of toxicology and environmental concerns, unable to provide fully satisfactory multi-metal corrosion protection and pitting resistance, or characterized by unacceptable physical properties.